Anion charge and lattice volume dependent lithium ion migration in compounds with fcc anion sublattices

Abstract

Proper design principles are essential for the efficient development of superionic conductors. However, the existing design principles are mainly proposed from the perspective of crystal structures. In this work, the face-centered cubic (fcc) anion sublattices were creatively constructed to study the effects of anion charge and lattice volume on the stability of lithium ion occupation and lithium ion migration by the density functional theory calculations. Both the large negative anion charges and large lattice volumes would increase the relative stabilities of lithium-anion tetrahedron, making lithium ions prefer to occupy the tetrahedral sites. For a tetrahedral lithium ion migration to its adjacent tetrahedral site through an octahedral transition state, the smaller the negative anion charge is, the lower the lithium ion migration barrier will be. While for an octahedral lithium ion migration to its adjacent octahedral site through a tetrahedral transition state, the more negative anion charge is, the lower the lithium ion migration barrier will be. New design principles for developing and optimizing superionic conductors with the fcc anion sublattice were proposed. Low lithium ion migration barriers would be achieved by adjusting the non-lithium elements within the same crystal structure to obtain the desired electronegativity difference between the anion element and the non-lithium cation element.